The terms “microsilica” and used in the specification and claims of this application refer to particulate, amorphous SiO2 obtained from a process in which silica (quartz) is reduced to SiO-gas and the reduction product is oxidised in the vapour phase to form amorphous silica. Microsilica may contain at least 70% by weight silica (SiO2), and preferably >97% and has a specific density of 2.1-2.3 g/cm3 and a surface area of 12-40 m2/g, typically 20 m2/g. The primary particles are substantially spherical and may have an average size of about 0.15 μm. Microsilica is preferably obtained as a co-product in the production of silicon alloys in electric reduction furnaces.
Microsilica slurry is currently used widely in construction applications such as fibre cement, concrete and in oil well cementing. Microsilica acts as pozzolanic material or inorganic binder by interacting with calcium hydroxide as a hydration product of Portland cement and water to improve the compressive strength of cement. In all of these applications, it has been demonstrated that microsilica used in slurry form performs much better than in powder form. Most commercially available microsilica slurries are normally supplied as 50 wt % microsilica slurries. Sulphuric acid is normally used to adjust the pH of the dispersion to lie in the range 4 to 7, to give stability to the dispersion.
Conventional microsilica slurry suffers from two challenges, namely, geletion and settling.
The stability of a microsilica slurry in terms of sedimentation and geletion depends mainly on the quality of the dry microsilica used for making the slurry. Since microsilica has a very small particle size, a slurry will show a high stability towards sedimentation when stored for long periods of time; more than 3 months for a well-dispersed slurry. However, when considering the stability of a microsilica slurry, a distinction should be made between particle settling and geletion.
Settling is a natural phenomenon for many inorganic dispersions such as silica, alumina or titania. According to Stoke's low, the main factors affecting settling are the low apparent viscosity of the continuous phase (water) and the size of the dispersed particles. The larger the dispersed particles, the faster the settling rate will be. Generally, a 50 wt % silica slurry with a SiO2 content >95 wt % has a high potential for settling due to the low apparent viscosity of <20 mPa·s at a shear rate of 20 s−1. Settling of coarse particles can be avoided by sieving the slurry or by increasing the viscosity of slurry by employing thickening agents such as Xanthen gum, cellulose, polyacrylate or nanosilica.
Geletion or network formation by the particles occurs when the particles are attracted to each other due to Van der Waals forces and/or chemical bridging by cations, creating a network structure which can have different forms and strengths. The main factor influencing geletion in the case of a microsilica slurry is contamination with other inorganic metal oxides such as K2O, Na2O, CaO, MgO, Al2O3 and Fe2O3. These oxides occur naturally in microsilica and dissolve to a certain extent into the water, providing the aqueous phase with different types of cations which tend to make bridges between the particles.
The bridging causes the microsilica particles to agglomerated or flocculate which results in a high viscosity and even gel or paste formation in some cases.
Generally, microsilica can be classified into three types according to the tendency to form a gel:    a) Not forming a gel would be microsilica with a total metal oxide (TMO) below 3 wt %.    b) Microsilica forming a gel (network) would be microsilica with a TMO larger than 3 wt %.    c) Microsilica forming a hard gel (network) would be with a TMO larger than 10 wt %.
These problems of settling and gelling are addressed in the present Applicants' EP 1534646, in which a polysaccharide is included in a slurry of water amorphous silica and silica flour, as a stabiliser. This measure is effective but it has now been found that in some instances, there is a deterioration of this slurry caused by biodegradation of the polysaccharide, due to bacterial attack. It is possible to avoid the biodegradation by adding biocides, but this is not a satisfactory solution as biocides could cause contamination of the environment during use of the slurry. For offshore oil industry the regulations demand biocide free products in order not to harm the aquatic organisms.
It is an object of the present invention to provide an aqueous slurry of amorphous silica which is stable for at least six months, both from the point of view of settling and gelling, and at the same time, to avoid the use of polysaccharides and other biodegradable additives.